1. Field of the Invention
The present invention relates to an organic optical gain device and a method of exciting the same.
More particularly, it relates to an organic optical gain device which has an improved threshold of excitation intensity, and higher optical gain, and a method of efficiently excitating said device.
2. Description of the Related Arts
Heretofore, ASE (Amplified Spontaneous Emission) device and Laser device provided with a thin film, as the substrate (in the form of sheet or rod), formed by dispersing a slight amount of fluorescent dye into a transparent or noncrystalline polymer such as PMMA (poly(methyl methacrylate)) have been known. Specific embodiments of these devices are disclosed in "Dye Lasers" edited by F. D. Schafer, Topics in Applied Physics Vol. 1, 3rd ed., pp. 82-83 (Springer Verlag) and literatures cited therein.
The above-mentioned devices excited, by N.sub.2 laser, a thin film formed by dispersing 8.times.10.sup.-3 mol/liter of Rhodamine 6 B into a polyurethane sheet at a thickness of 0.8 .mu.m to realize an optical gain of 100 dB/cm (see Appl. Phys. Lett. reported by H. P. Weber and R. Ulrich, 1971, vol. 19, p.38).
Such a device, however, has a threshold of excitation intensity for ASE light as high as 100 kW/cm.sup.2, and has been required for improvement.
If the threshold is lowered, the efficiency of ASE or lasing is improved, and also the optical gain caused by excitation power is increased.
In conventional devices, since the quantum yield of fluorescence of 0.1 or more is obtained only under a condition of dye distribution as dilute as 10.sup.-3 to 10.sup.-2 mol/liter so that inversion distribution is caused, it has been impossible to lower the threshold of excitation intensity. This is because the excitation light is not effectively absorbed through a thin film with a dilute distribution of dye. Also, conventional devices have disadvantages in that gain per unit optical-path is small due to dilute distribution of dye.
Generally, when excitation is caused optically, it is necessary for an excitation light to have an absorption coefficient of 10.sup.4 cm.sup.-1 or more to be absorbed completely in an active layer at a thickness of 1 .mu.m or less. Since the absorption cross section of dye is usually about 10.sup.16 cm.sup.2, the layer must contain 10% or more by weight of dye in order to absorb an excitation light completely. Accordingly, conventional dilute dispersion systems have not been able to effectively utilize stripe excitation or spot excitation.
Moreover, formerly, if 10% or more by weight of the conventional fluorescent dye was contained, it caused quenching by aggregation, and the quantum yield of fluorescence decreased drastically, and hence such a device could not be used for ASE elements. Further, when the active layer is an ultra thin film, optical waveguiding is sometimes impossible due to difference in refractive index between two clad layers.